Ok, i'll add my $1.87 (price of gas today)... Downhill racing is expensive, plan on braking alot of things, including yourself, rims, frames, drivetrains ect. Your current bike looks like a nice XC bike, start racing that in your local races, ask your LBS about the local scene. If you want to get into downhill, you need to know how to handle your bike, downhill or not. You just cant jump into the sport, its going to take alot of years to get the skills to race DH.
Now, if you just want to race downhill for fun, get a good full suspension bike that climbs well, so you can ride upthe trail before coming down, dont plan on racing anyone, or pushing your limits too far.
Im not sure about this bike your friend wants to sell you, whats wrong with it? what does it come with, does it fit you? just dont purchase a bike becouse its cheap!
More later... any questions?